The present invention relates to an internal thread producing tool, which can be either a cutting screw tap or a non-cutting thread former. Conventionally the tool is either of high-speed steel or carbide metal. The invention also pertains to a method of forming an internal thread.
Shown in FIG. 1 is a conventional screw tap which is a cutting tool for cutting an internal thread. The tap includes a rear shank 1 for being clamped in a tool support, and a front thread cutting part, the so-called chamfer a, consisting of screw turns. The chamfer is arranged on the free end of the tool and is interrupted by longitudinal grooves for chip removal. Disposed rearwardly of the chamfer is the so-called guide or guide part b, which also consists of screw turns interrupted by grooves for chips. The guide part does not perform any cutting work, however, but serves, as the name implies, for the guiding and the uniform driving of the screw tap through the hole to be tapped. Tools producing internal thread without the use of a guide part have also been proposed recently for rigid tapping, see Swedish Patent Application 9600927-9.
In principle there are two basic types of screw taps, namely the bottoming tap (also called blind hole tap) and the straight-through tap. As the names imply, holes open on one side and closed on the other side by a bottom are involved in the first case, while bottomless holes open on both sides are involved in the second case. In the first case the chips have to be conveyed out of the hole, in the opposite axial direction to that of the tapping. The grooves for chips are consequently formed helically in the same direction as the direction of rotation of the tap, whereby they have a chip-conveying effect. Known bottoming taps are described e.g. in DE-U-86 23 509.5, Marburger U.S. Pat. No. 4,462,727 and Von Holst et al. U.S. Pat. No. 5,487,626.
In the second case (i.e. with straight-through taps) it is more favorable if the chips are conveyed to the front, through the hole, in the same direction as the tapping direction. In this case, therefore, the grooves for chips are formed helically opposite the direction of rotation of the tap and thereby have the effect of driving the chips towards the tap tip. Examples of straight-through screw taps are described for example in DE-A-3 419 850 and DE-U-83 24 835.8.
Also known are screw taps with straight, axially extending grooves for chips, which in principle convey the chips neither to the front nor to the rear. These are particularly suitable with short-chipping material and with small thread depths. Three different types of taps with straight-line grooves are often employed, namely, a taper tap, a second tap and a plug tap.
Unlike the screw tap, thread formers do not produce any chips; they simply deform the material. With thread formers also it is the practice to speak of a "chamfer" and a guide part, although the term chamfer is strictly speaking incorrect, since thread formers do not cut. For the sake of simplicity, however, this expression will also be used here for describing the tapering front part, in which, as in the case of screw taps, the work (albeit deforming work) for producing the thread is performed. A thread former according to the state of the art is described, for example, in DE-A-2 414 635.
It is normal to use considerable amounts of coolants and lubricants when operating the thread-producing tools described above. In recent times, however, a strong trend in general towards dry machining can be noted, not only in the production of thread but also, or even in particular, in turning, drilling and milling. The main reasons for this are to save on the costs of cooling lubricants and to protect the environment.
Various machining processes can be converted to dry chipping relatively easily. The latter include, e.g.,turning and milling. Other processes present far greater problems when they are carried out dry screw tapping and thread forming are particularly problematical. In such cases the so-called minimal lubrication technique is often used as a possible solution wherein minimal lubricant is fed via nozzles from the outside or centrally through the spindle and the tool. It is therefore often necessary for the screw tapping process (or thread forming process), to install a minimal lubrication system on the machine. This naturally complicates the machine and makes it more expensive.
The invention is consequently based on the aim of saving on cooling lubricant costs and protecting the environment.
It is further an object of the invention to avoid the need for the installation of a minimal lubrication system on a thread-producing machine.